Process of Producing a Ceramic Matrix Composite

ABSTRACT

A process of producing a ceramic matrix composite (CMC) is provided the steps of preparing a ceramic material having a plurality of pores as a CMC substrate; heating a metal material to be molten wherein the metal material has a melting point lower than the CMC substrate and has a high activity; adding the CMC substrate to the molten metal material so that the molten metal material enters the pores of the CMC substrate to occur chemical reactions; removing the CMC substrate filled with the molten metal material; and cooling the removed CMC substrate filled with the molten metal material to form a CMC having a plurality of metal grains. Plain strain fracture toughness (K IC ) of typical ceramic S26 is 4.53 MPa m 1/2 . As a comparison, CMC has K IC  of 21.11 MPa m 1/2  about 466% of ceramic S26.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to ceramic materials and more particularly to aprocess of producing a ceramic matrix composite (CMC) having increasedfracture toughness, increased heat-resistance, increased electricalconduction, and increased thermal conduction.

2. Description of Related Art

Materials technology is advancing rapidly in recent years. Thetraditional structure provided by the application of metals to replaceceramics has been an increasing tendency. Ceramic materials have thefollowing characteristics superior to metals: High elasticitycoefficient/weight ratio, high hardness, high compressionstrength/weight ratio, high/low thermal conductivity, low thermalexpansion coefficient, high melting point, and good corrosion/oxidationresistance. These characteristics of the ceramic materials make thesematerials gradually favored by the industry. Ceramic materials arewidely employed in special optical, electrical, thermal and mechanicalfields, and many different kinds of ceramic products are developed forproducing consumer products.

For example, ceramic knives are made of zirconium dioxide and haveproperties similar to natural diamonds. The blade can be extremelysharp, hard, and abrasion resistant. Further, the blade surface is denseand is not subject to contamination by food juice. Furthermore, it iseasy to clean, which can reduce the growth of microorganisms, and isextremely light. Thus, ceramic knives have become a good house helper.

Electrical conduction of ceramic materials is activated by heating orother methods to generate free electrons In addition, an electric fieldis applied to the ceramic material. As an end, the electrical conductionis generated. A typical ceramic material having electrical conduction isSiC which has a maximum operating temperature of 1,450 degrees Celsius.Similarly, MoSi2 has a maximum operating temperature of 1,650 degreesCelsius. Novel ceramic materials having electrical conduction includezirconium dioxide having a maximum operating temperature of 2,000degrees Celsius, and thorium oxide having a maximum operatingtemperature up to 2,500 degrees Celsius.

An ion-conducting ceramic material is a molten electrolytic solution orelectrolyte having high ion conductivity similar to the solid ceramicmaterial. β alumina ceramic material is a typical cationic conductor. Itmainly relies on the migration of sodium ions for being conductive.Zirconium dioxide based ceramic material is an anionic conductivematerial and relies mainly on migration of oxygen anions for beingconductive. Ion-conducting ceramic materials can also be used to producea number of novel solid-state batteries such as sodium-sulfur batteries.It may be applied to electric power supply (e.g., battery) forautomobile in the future. As described above, ceramic products not onlychange the traditional manufacturing processes but also deeply affectour daily lives.

While ceramic materials have become the darling of materials, it hasinherent shortcomings such as excessive brittleness. Particularly, theytend to fracture when tensile stress is concentrated on a portionthereof. As a result, it leads to failure. Fracture toughness of typicalceramic materials is shown in FIG. 1.

Bonding of ceramic materials is either covalent or ionic. Covalentbonding between atoms is formed by shared and overlapping valenceelectrons. Ionic bonding is formed by transferring electron(s) from acation to an anion. Thus, the covalent electron cloud in a covalentbonded ceramic material does not form a bond due to atoms displacementfrom each other in response to an applied force. Similarly, breaking theionic bonding between an ionic ceramic material will result in alladjacent atoms becoming either all cations or anions and generate arepulsive force to cause rupture. Consequently, ceramic materials arebrittle in nature.

Regarding metals and polymeric materials, use of these materials doesnot cause fracture or breakage as long as the applied force is lowerthan the ultimate tensile strengths of the materials. Further, eventhese materials are overloaded beyond their yield strengths, significantplastic deformation usually occur prior to the final failure and serveas a warning. Because ceramic materials are brittle and have poortoughness, they are subject to sudden failure. Therefore, industrialapplications of ceramic materials are significantly limited.Consequently, it is desired to develop a tough ceramic matrix compositemaintaining the attractive characteristics but without the disadvantagesof conventional ceramic materials.

SUMMARY OF THE INVENTION

It is therefore one object of the invention to provide a process ofproducing a ceramic matrix composite (CMC) comprising the steps ofpreparing a ceramic material having a plurality of pores as a CMCsubstrate; heating a metal material to be molten wherein the metalmaterial has a melting point lower than the CMC substrate and has a highactivity to react with the component of the CMC substrate; adding theCMC substrate to the molten metal material so that the molten metalmaterial enters the pores of the CMC substrate to occur chemicalreactions; removing the CMC substrate filled with the molten metalmaterial; and cooling the removed CMC substrate filled with the moltenmetal material to form a CMC having a plurality of metal grains.

The above and other objects, features and advantages of the inventionwill become apparent from the following detailed description taken withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a table showing fracture toughness of typical kinds of ceramicmaterials;

FIG. 2 is a flow chart illustrating a process of producing a ceramicmatrix composite according to the invention;

FIG. 3 is a perspective view of a CMC article formed by the process;

FIG. 4A is an enlarged view of the circle in FIG. 3;

FIG. 4B is another enlarged view of the circle in FIG. 3; and

FIG. 5 is a table showing maximum fracture load and fracture toughnessof CMC of the invention and S26 ceramic material of the prior art.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 2, a process of producing a ceramic matrix compositein accordance with the invention is illustrated below.

In step 10, a porous ceramic material is prepared as a CMC substrate.

In step 20, a metal material is heated to be molten.

In step 30, the CMC substrate is added to the molten metal material sothat the molten metal material may enter pores of the CMC substrate tooccur chemical reactions.

In step 40, the CMC substrate filled with molten metal material isremoved and cooled to form a CMC having a plurality of metal grains.

Referring to FIGS. 3, 4A, 4B, and 5, a CMC article is schematicallyshown. The CMC article comprises a CMC substrate 1 and a plurality ofmetal grains 2 filled in the pores of the CMC substrate 1. The CMCsubstrate 1 is formed of alumina, silica, zirconium dioxide, siliconcarbide, copper oxide, or silicon nitride. The metal grains 2 have amelting point lower than the CMC substrate 1 and have the characteristicof high activity. The metal grains 2 may be aluminum, nickel, tin,magnesium, beryllium, chromium, iron, zinc, zirconium, copper, ortitanium and their alloys. The CMC substrate 1 of the invention is acomposite and is conductive.

The CMC having the CMC substrate 1 of the invention can be subject toheat treatment to form as a thermal conductive but electricallynon-conductive ceramic composite.

The CMC having the CMC substrate 1 of the invention can be subject toheat treatment so that the metal such as aluminum can be oxidized toform as a corrosion proof ceramic composite.

The CMC having the CMC substrate 1 of the invention can be subject to astep of removing portions of the metal grains 2 in the pores by heatingto a molten state or by etching with chemicals such as acids so as toform as a ceramic composite having metal residues 3 which are left inthe pores near the surface of the CMC. The ceramic composite has theeffect of absorbing lubricant and thus can be made into ball bearings orself-lubricating bearings.

In brief, the CMC of the invention of the composite ceramic substrate isproduced by filling the molten metal material into the pores of the CMCsubstrate 1, and causing chemical reactions to occur by replacing thelattice arrangement of ceramic material. As a result, the CMC of theinvention has the benefits of significantly increased fracturetoughness, heat-resistance, electrical conduction, and thermalconduction.

In FIG. 5, maximum fracture load and fracture toughness of CMC of theinvention and S26 ceramic material of the prior art are shown forcomparison. Plain strain fracture toughness (K_(IC)) of typical ceramicS26 is 4.53 MPa m^(1/2). As a comparison, CMC has K_(IC) of 21.11 MPam^(1/2) about 466% of ceramic S26.

While the invention has been described in terms of preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with modifications within the spirit and scope of theappended claims.

What is claimed is:
 1. A process of producing a ceramic matrix composite(CMC) comprising the steps of: preparing a ceramic material having aplurality of pores as a CMC substrate; heating a metal material to bemolten wherein the metal material has a melting point lower than the CMCsubstrate and has a high activity to react with the component of the CMCsubstrate; adding the CMC substrate to the molten metal material so thatthe molten metal material enters the pores of the CMC substrate to occurchemical reactions; removing the CMC substrate filled with the moltenmetal material; and cooling the removed CMC substrate filled with themolten metal material to form a CMC having a plurality of metal grains.2. The process of claim 1, wherein the CMC substrate is formed ofalumina, silica, zirconium dioxide, silicon carbide, copper oxide orsilicon nitride.
 3. The process of claim 1, wherein the metal materialis aluminum, nickel, tin, magnesium, beryllium, chromium, iron, zinc,zirconium, copper, or titanium and their alloys.
 4. The process of claim1, wherein the CMC is subject to a step of removing portions of themetal grains by heating to a molten state or by etching with chemicalssuch as acids to form as a ceramic composite having a plurality of metalresidues left in the pores near the surface of the CMC, the ceramiccomposite being capable of absorbing lubricant and being made into aball bearing or a self-lubricating bearing.